Power converter using soft composite magnetic structure

ABSTRACT

A power conversion device includes a magnetic core; and a plurality of windings surrounding portions of the magnetic core, including a first winding and a second winding magnetically coupled through the magnetic core. The magnetic core comprises a first part formed of a first material and a second part formed of a second material, the first material having a first stiffness and the second material having a second stiffness substantially less than the first stiffness. The first winding and the second winding are magnetically coupled through the first part of the magnetic core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 13/076,923,titled “POWER CONVERTER USING ORTHOGONAL SECONDARY WINDINGS” and filedconcurrently with the present application, which is incorporated hereinby reference.

BACKGROUND

Some power conversion systems, for example, dual interleaved boost powerfactor converter (PFC) systems, make use of magnetically coupledinductors or coils wound around a magnetic core. For instance, referringto FIG. 1A, a magnetic core 150 of a boost converter includes a firstset of inductor coils 152 and a second set of inductor coils 154.Inductor coils 152 are disposed around a first leg 156 of a core 158,and inductor coils 154 are disposed around a second leg 160 of the core.Energy storage in boost converter 150 is localized in a center leg 162including a gap 164. The magnetic (H) field in gap 164 is orientedperpendicular to the wide axis of inductor coils 152 and 154. Boostconverters including magnetic core 150 are generally suitable for apower throughput of about a few hundred Watts. However, attempts toscale up such boost converters may face efficiency limitations in somesystems, for instance due to geometry constraints and/or eddy currentlosses when the power rating is increased beyond the 1 kW range. In someexamples, the power conversion systems are used in professional soundsystems.

Some power conversion systems make use of one primary winding andmultiple secondary windings, with one of the secondary windings beingused to provide “housekeeping” power to control circuitry, such that aseparate power conversion component is not needed to power the controlcircuitry. In situations in which the control circuitry requires powerduring standby periods, the production of housekeeping power may beinefficient, for example, due to losses in the switching components ofthe system driven by the primary windings. This inefficiency negatessome of the advantages of sharing the primary windings for multiple setsof secondary windings during normal operation.

Referring to FIG. 1B, an example of a dual interleaved boost convertercircuit 100 includes inductors L1 112 and L2 114, which are magneticallycoupled across a common core 102. The degree of coupling between theinductors is controlled by the width of the gap separating the windingsof the two inductors. The maximum flux ripple in the core of the dualinterleaved boost converter circuit 100 is roughly half that of a singleboost circuit, and the AC ripple on the dual interleaved boost circuitis also reduced. Two switches Q1 122 and Q2 124 (e.g.,metal-oxide-semiconductor field effect transistors (MOSFETs)) are dutycycle controlled and typically run 180° out of phase, although in somecases 90° operation may be preferable. A circuit having sufficientlycoupled inductors exhibits little to no ripple current.

SUMMARY

In a general aspect, a power conversion device includes a magnetic core;and a plurality of windings surrounding portions of the magnetic core,including a first winding and a second winding magnetically coupledthrough the magnetic core. The magnetic core comprises a first partformed of a first material and a second part formed of a secondmaterial, the first material having a first stiffness and the secondmaterial having a second stiffness substantially less than the firststiffness. The first winding and the second winding are magneticallycoupled through the first part of the magnetic core.

Embodiments may include one or more of the following.

The first material has a first magnetic permeability and the secondmagnetic material has a second magnetic permeability less than the firstmagnetic permeability.

The first material comprises ferrite.

The second material comprises a composite. The second material includesa polymer. The second material includes at least one of iron powder,ferrite powder, Sendust, Metglass powder, or an amorphous soft magneticalloy.

The second stiffness is about 1000 times less than the first stiffness.The second stiffness is less than about 100 MPa.

The first winding is disposed on a first substrate and the secondwinding is disposed on a second substrate, the magnetic core passingthrough openings in the first substrate and the second substrate.

The first part of the magnetic core comprises a first element and asecond element. The first element includes a plurality of first legs,each first leg configured to fit through a corresponding opening in thefirst substrate. The second element includes a plurality of second legs,each second leg configured to fit through a corresponding opening in thesecond substrate. The first legs and the second legs mate to form thefirst part of the magnetic core.

The first substrate is a first circuit board and the second substrate isa second circuit board.

The second part of the magnetic core comprises a third element, thethird element coupled in contact with the first part of the magneticcore. At least a portion of the third element is disposed between thefirst winding and the second winding.

The second part of the magnetic core forms an annular structure.

The device further includes circuitry coupled to the windings forming apower converter. The first winding and the second winding form coupledinductors. The power converter comprises a dual interleaved boostconverter. During operation of the boost converter, the first windingand the second winding form coupled inductors and during operationcyclical energy storage in the magnetic core is substantiallyconcentrated in the second part of the magnetic core.

In another general aspect, a method for assembling a power conversiondevice includes assembling a magnetic core having a first part formed ofa first material and a second part formed of a second material, thefirst material having a first stiffness and the second material having asecond stiffness substantially less than the first stiffness. Assemblingthe magnetic core includes disposing a first element of the first partof the core within a first winding; and forming the second part of thecore to maintain contact with the first part of the core such that thesecond part of the core forming at least part of magnetic flux pathsinduced by current in the first winding.

Embodiments may include one or more of the following.

The first material has a first magnetic permeability and the secondmaterial has a second magnetic permeability less than the first magneticpermeability.

The method further includes disposing a second element of the first partof the core within a second winding. The second part of the coreproviding magnetic coupling between the first winding and the secondwinding. The method further includes mating the first element and thesecond element of the first part of the core; and forming the secondpart to maintain contact with the first part after mating the first andthe second elements.

The first winding is formed on a first substrate, and wherein disposingthe first element of the first part of the core within a first windingincludes passing the first element through one or more openings in thefirst substrate.

The second part comprises a mechanically soft material, and forming thesecond part to maintain contact with the first part includes deformingthe second part.

Forming the second part to maintain contact with the first part includespositioning a precursor of the second material in a region defined atleast in part by the first part of the core; and causing atransformation of the precursor material to form the second material.Causing the transformation comprises curing the precursor material. Theprecursor material comprises a liquid material, and wherein position theprecursor comprises pouring the precursor into the region.

In a general aspect, a power conversion device includes a magnetic core;and a plurality of windings surrounding portions of the magnetic core,including a first set of windings defining a first magnetic flux path, asecond set of windings defining a second magnetic flux path magneticallyorthogonal to the first magnetic flux path, and a third set of windings.Each winding of the third set of windings is configured to be excitablevia both the first flux path and the second flux path.

Embodiments may include one or more of the following.

The device is operable in a plurality of modes, including a first modein which power is transferred from one or more windings of the first setof windings to the windings of the third set of windings, and in asecond mode in which power is transferred from windings of the secondset of windings to the windings of the third set of windings.

The device further includes circuitry to form a first power supply usingthe first set of windings operable only in the first mode, and circuitryto form a second power supply for providing power via the third set ofwindings operable in both the first mode and the second mode.

The first mode comprises a primary operating mode and the second modecomprises a standby operating mode. The first power supply has a powercapacity as least ten times greater than the second power supply. Thefirst power supply has a power capacity of at least 0.2 kW.

The first power supply comprises a boost converter. The first set ofwinding comprises a plurality of windings coupled by the magnetic core,and wherein the boost converter comprises an interleaved boostconverter.

The circuitry further comprises a rectifier coupled to each of thewinding of the third set of windings. The circuitry further comprises acharge pump coupled to each of the winding of the third set of windings.

The second set of windings comprises a plurality of windings arranged ina serial connection, and the third set of windings comprises a pluralityof windings. Each winding of the third set of windings corresponds to adifferent one of the windings of the second set of windings.

Each of the windings of the first set of magnetic windings is coupled toa MOSFET having a first current rating, and wherein each winding of thesecond set of windings is coupled to a MOSFET having a second currentrating less than the first current rating.

Each winding of the first set of winding is disposed on a substrate of aset of one or more substrates, the magnetic core passing throughopenings in the substrates.

In another general aspect, a method for power conversion includes, in afirst operating mode, exciting windings of a first set of windingssurrounding portions of a magnetic core causing a first power output.The exciting of the first set of windings causing a second power outputvia a third set of windings magnetically coupled to the first set ofwindings via the magnetic core. The method further includes, in a secondoperating mode, exciting winding of a second set of windings surroundingportions of the magnetic core, a second magnetic flux path formed by thesecond set of windings being magnetically orthogonal to a first magneticflux path formed by the first set of windings. The exciting of thesecond set of windings causing a power output via the third set ofwindings magnetically coupled to the second set of windings via themagnetic core.

Embodiments may include one or more of the following.

The first operating mode comprises a primary operating mode and thesecond operating mode comprises a standby operating mode.

The first set of windings form part of a first power supply and thesecond set of windings form part of a second power supply. The firstpower supply has a power capacity at least ten times greater than thesecond power supply.

Among other advantages, the systems and methods described herein providea scalable geometry that allows a dual interleaved boost converter tooperate at the multi-kW level without significant AC conductor losses.In general, a system having coupled inductors has low core losses due toa reduced AC flux component in the core, and a smoother current and lessloss in switches due to a reduced RMS current flowing in the switches.

The use of a material in the boost converter that is both magneticallyand mechanically soft allows strict manufacturing tolerances to beachieved without a gap between materials and without the generation ofthermal stresses or cracks. In some cases, the boost converter can befabricated at room temperature, allowing the fabrication process to bereadily integrated with existing manufacturing processes.

A boost converter having the ability to generate standby power withoutdriving the large loads coupled to the main primary induction coilsreduces switching losses and other power inefficiencies.

Other features and advantages of the invention are apparent from thefollowing description and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a side cross-sectional view of a boost converter.

FIG. 1B is a circuit diagram of a dual interleaved boost circuit.

FIG. 2 is a side cross-sectional view of a boost converter having amagnetic core formed from two materials.

FIGS. 3A and 3B are perspective views of the boost converter of FIG. 2as exploded and as assembled, respectively.

FIG. 4 is a circuit diagram of a boost circuit with low-power windings.

FIGS. 5A and 5B are side cross-sectional views of a boost converted withcoils for low power driven by main power coils and driven by low powercoils, respectively.

FIGS. 6A and 6B are top cross-sectional views of the boost convertersshown in FIGS. 5A and 5B, respectively.

FIG. 7 is a perspective view of a boost converter configured for normaland standby operation.

DETAILED DESCRIPTION

Referring to FIGS. 2 and 3A-B, in some embodiments, the dual interleavedboost circuit 100 shown in FIG. 1 is implemented using a magnetic coremade up of multiple elements 210, 212, 214 that are assembled togethersurrounding the windings 112, 114 of the inductors. Winding 112 isformed on a printed circuit board 222 around a central opening 232 ofthe board. Winding 114 is similarly formed around a central opening 235of a second printed circuit board 224. As shown in the exploded view ofFIG. 3A, the magnetic core is formed in part by a first multi-leggedelement 212, the legs of which, when assembled as shown in FIG. 3B, passthrough openings 231, 232, 233 in the first circuit board 222. Themagnetic core is further formed by a second multi-legged element 214,the legs of which, when assembled, pass through openings 234, 235, 236in the second circuit board 224 such that the legs of elements 212 and214 mate between the two printed circuit boards. When assembled, each ofthe windings 112, 114 is effectively wound around the mated center legsof the first and second elements 212, 214. As shown in thecross-sectional view of FIG. 2, the mated first and second elements 212,214 do not fill the area between the windings.

In general, elements 212 and 214 are made of a material with a highmagnetic permeability (relative permeability μ), such as ferrite, toenable magnetic coupling between the windings. In the area between thewindings, a low-μ material allows for energy storage, which isproportional to μ⁻¹. In some embodiments, the magnetic core is furtherformed from a third element 210 of a different material than the firsttwo elements. Referring to FIG. 3A, in some embodiments, this thirdelement 210 forms a ring or “donut” shape (e.g., a rectangular ring or asubstantially circular ring) such that, when assembled, the thirdelement fills the space between the windings where the air gap wouldhave been.

In some cases, the material of the third element 210 has a significantlylower magnetic permeability μ (i.e., higher magnetic reductivity) thanthe first 212 and second 214 elements. Referring again to thecross-sectional view of FIG. 2, the magnetic field lines 202, 204induced by corresponding currents in the windings 112, 114 are largelycoupled via the high-permeability elements 212, 214, while energystorage is primarily localized to the lower-permeability ring element210.

In some embodiments, the first and the second core elements 212, 214 areformed of a mechanically hard, magnetically soft material, such asferrite (which has an elastic modulus of about 100 GPa). These parts maybe difficult to manufacture to high dimensional tolerance or to maintainat a precise dimension due to environmental factors (e.g., temperature).For example, the lateral distance between the legs of the elements maynot be fabricated to a predictable precise dimension.

In some embodiments, the third ring element 210 of the core is formedfrom a mechanically soft material having an elastic modulus of about1000 times less than the elastic modulus of the rigid material of thefirst and second core elements. For instance, in some embodiments, themodulus of the third ring element is limited to no more than about 100MPa. An example of a suitable type of material is a soft, pliablecomposite combining a magnetic phase (e.g., iron powder, ferrite powder,Sendust, or another finely ground magnetically soft material capable ofproviding low hysteresis and eddy current losses) in a polymer matrix(e.g., a rubber, an epoxy, or a urethane). The third ring element has amagnetic permeability in the range of about 8-80, or preferably in therange of about 10-30. One example of such a material, made by DaidoSteel Co., Ltd. (Tokyo, Japan), is a composite of a Metglas® alloy(Metglas, Inc., Conway, S.C.) in a rubber matrix that exhibits AC lossesclose to that of powdered iron (μ=10) and has a permeability μ=30 atzero field.

In general, the mechanically hard components (i.e., circuit boards 222,224 and hard elements 212, 214 of the core) are assembled using standardmanufacturing processes. In some embodiments, the components areassembled leaving a gap into which ring element 210 can later beinserted rather than the ring being inserted during initial assembly. Insome embodiments, an uncured precursor to the material of ring element210 is squeezed into the gap and cured at elevated temperature to formthe ring element. In other embodiments, ring element 210 is formedoutside of the boost converter and mechanically deformed as it is pushedinto the gap or as the other elements of the boost converter are pushedaround the ring element. In some examples, the ring element 210 isformed of a putty-like material. To be compatible with existingmanufacturing protocols, room temperature fabrication and assembly ofboost converter, including insertion of third element 210 of the core,is preferable.

In some examples, the third element 210 is formed from a material thatcures during or after the manufacturing process. For example, thematerial may be soft during assembly and then hardened in a curingprocess. In general, even in its hardened state, the material of thethird element remains mechanically softer (e.g., about 1000 timessofter) than the material of the mechanically hard elements of the core)so that any strain resulting from unequal coefficients of thermalexpansion is absorbed by the third element. In some examples, thematerial is resilient, thereby maintaining contact with the otherelements of the core in the face of mechanical movement or thermalexpansion of the elements. In some examples, a chamber is formed betweenthe circuit boards, and the third element is formed by pouring orinjecting a liquid into the chamber, which may then be cured to form aflexible or rigid third element. In some examples, the manufacturingprocess is performed at a high temperature at which the third element issoft (e.g., flexible, resilient), while in operation the device operatesat a lower temperature at which the element is relatively harder (e.g.,less flexible or resilient).

In other examples, the third ring element 210 of the core may be formedfrom a rigid material. However, if the first and second core elementsare not precisely dimensioned or if the third element exhibits differentthermal expansion characteristics than the first and second elements, arigid third element may have to be under-sized sufficiently to allowassembly. Such under-sizing may result in an undesirable air gap.Furthermore, if the ring element 210 were formed of a rigid materialhaving a substantially different coefficient of thermal expansion thanthat of the other elements, cracking or distortion may occur uponheating of the boost converter.

Referring to FIG. 3A, the windings of the boost converter are formedusing printed circuit tracings on the printed circuit board (e.g., board222). For instance, the windings can include spiral paths on one or morelayers of the board with the paths surrounding openings in the boardthrough which the magnetic core passes when assembled. Note that thevertical dimension of trace is very small as compared to its horizontaldimension, thereby forming a ribbon-like conductor. In certain modes ofoperation, such a low vertical dimension combined with the direction ofmagnetic field lines reduces eddy current losses as compared to otherconfigurations. In part, the reduced losses are due to the orientationof the magnetic field in the windings: the magnetic field (H) vector isparallel to the wide axis of the windings. The reduced losses are alsodue to the magnetic permeability of the winding material: the magnetic(H) field is about ten times lower in a material with μ=10 than in air,and eddy current losses scale as the magnetic field squared (H²).

The boost converter of FIGS. 2 and 3A-B may provide for reduced ACconductor losses, such as eddy current losses, as compared to a boostconverter having an air gap, for instance by roughly a factor of 5,under the same operating conditions. This reduction in AC conductor lossmay bring the AC conductor loss closer to the level of the DC loss(e.g., roughly twice the DC loss), which may be desirable in manyapplications.

In a power factor converter (PFC) such as the boost converters describedabove, the PFC windings provide a high level of power to a load. Forinstance, the PFC windings in boost converter 100 (FIG. 1) are coupledto a pair of large MOSFET switches 122, 124 (Q1 and Q2) which enable thePFC windings to throughput kilowatts of power.

When a PFC is delivering little or no power to the load, it may in somecases still be desirable to maintain a low level of power for standbyoperation. A set of secondary windings may be used to provide low power,enabling standby operation. However, driving the smaller secondarywindings using the voltage across the larger primary windings (i.e.,main primary windings 112, 114) may entail significant switching lossesresulting from the drain-source capacitance of the large MOSFET switches122, 124 (Q1 and Q2) coupled to the primary windings.

Referring to FIG. 7, a PFC choke 600 includes three sets of windings:main primary windings L1 112 and L2 114, which function as describedabove; secondary windings L4 608 and L5 609; and primary low-powerwindings L3A 610A and L3B 610B. Secondary windings 608, 609 and primarylow-power windings 610A, 610B are wound around outer legs 702, 704 of acore 700. Secondary windings 608, 609 provide efficient low-power output(e.g., housekeeping power) in both normal and standby operating mode,and can be excited either by currents in the main primary windings 112,114 (e.g., during normal operating mode) or by currents in the primarylow-power windings 610A, 610B (e.g., during standby operation). In someembodiments, main primary windings 112, 114 have a power capacity atleast ten times greater than the power capacity of main low-powerwindings 610A, 610B, e.g., a power capacity of at least 0.2 kW.

Referring to FIGS. 5A and 6A, magnetic flux lines 202, 204 are createdby a current through main primary windings 112, 114 (the arrows in FIG.6A indicate the direction of current flow in the windings). Flux lines202, 204 are capable of exciting secondary windings 608, 609, e.g.,during normal operating mode. Notably, flux lines 202, 204 induce anelectromagnetic field in primary low-power winding 610A that is out ofphase from the electromagnetic field induced in primary low-powerwinding 610B, such that the overall field cancels and no net voltage isinduced across the primary low-power windings. That is, main primarywindings 112, 114 are incapable of exciting a current in primarylow-power windings 610A, 610B.

Referring now to FIGS. 5B and 6B, a magnetic flux path 602 is created bya current through primary low-power windings 610A, 610B. Flux path 602is capable of exciting secondary windings 608, 609, e.g., during standbyoperation. Notably, none of the flux associated with primary low-powerwindings 610A, 610B links main primary windings 112, 114. That is,primary low-power windings 610A, 610B are incapable of exciting acurrent in main primary windings 112, 114.

Main primary windings 112, 114 and primary low-power windings 610A, 610Bare thus magnetically orthogonal to each other. That is, there is nomagnetic coupling between these two sets of windings, and each set ofwindings can be operated independently without inducing currents in theother set of windings.

Referring FIG. 4, in the circuit of PFC choke 600, secondary windings L4608 and L5 609 are coupled through rectifier circuits 424 (e.g., avoltage double rectifier) to provide a low voltage output. Generally, ina normal operating mode, these low power windings are coupled to andreceive power from the main primary inductor windings L1 112 and L2 114,as described above. In standby mode, the main windings are not energizedand the secondary windings L4 608 and L5 609 receive power from primarylow-power windings L3A 610A and L3B 610B, which are connected in seriesand coupled to a MOSFET switch 420 (Q3). That is, while the secondarywindings 608, 609 are rectified separately from the main primarywindings 112, 114 and from the primary low-power windings 610A, 610B,the secondary windings can be excited either by currents in the mainprimary windings 112, 114 or by currents in the primary low-powerwindings 610A, 610B, depending on the operating mode.

Because Q3 (the switch coupled to primary low-power windings 610A, 610B)is significantly smaller than Q1 and Q2 (of the switches 122, 124coupled to the main primary windings), driving the secondary windingswith the primary low-power windings during standby mode avoids theinefficiencies inherent in unnecessarily driving the large loads (Q1 andQ2) coupled to the main primary windings.

The use of a soft magnetic material, such as a magnetic composite, andthe orthogonality of primary and secondary magnetic windings are notlimited to use in PFC converter systems, but may used generally in,e.g., any power converter system or transformer having coupled inductorwindings.

It is to be understood that the foregoing description is intended toillustrate and not to limit the scope of the invention, which is definedby the scope of the appended claims. Other embodiments are within thescope of the following claims.

What is claimed is:
 1. A power conversion device comprising: a magneticcore; and a plurality of windings surrounding portions of the magneticcore, including a first winding and a second winding magneticallycoupled through the magnetic core; wherein the magnetic core comprises afirst part formed of a first material in direct contact with a secondpart formed of a second material, the first material having a firststiffness and the second material having a second stiffnesssubstantially less than the first stiffness; and wherein the firstwinding and the second winding are magnetically coupled through thefirst part of the magnetic core; and wherein the second part of themagnetic core at least partially fills the space in between windingmaterial of the first winding and winding material of the secondwinding; and wherein the first winding is disposed on a first substrateand the second winding is disposed on a second substrate, the magneticcore passing through openings in the first substrate and the secondsubstrate; and wherein the second part of the magnetic core isconfigured to surround a column of the first part of the magnetic core.2. The device of claim 1, wherein the first material has a firstmagnetic permeability and the second material has a second magneticpermeability less than the first magnetic permeability.
 3. The device ofclaim 1, wherein the first material comprises ferrite.
 4. The device ofclaim 1, wherein the second material comprises a composite.
 5. Thedevice of claim 4, wherein the second material includes a polymer. 6.The device of claim 4, wherein the second material includes at least oneof iron powder, ferrite powder, Sendust, or an amorphous soft magneticalloy.
 7. The device of claim 1, wherein the second stiffness is about1000 times less than the first stiffness.
 8. The device of claim 1,wherein the second stiffness is less than about 100 MPa.
 9. The deviceof claim 1, wherein the first part of the magnetic core comprises afirst element and a second element, the first element includes aplurality of first legs, each first leg configured to fit through acorresponding opening in the first substrate, the second elementincludes a plurality of second legs, each second leg configured to fitthrough a corresponding opening in the second substrate, and the firstlegs and the second legs mate to form the first part of the magneticcore.
 10. The device of claim 1, wherein the first substrate is a firstcircuit board and the second substrate is a second circuit board. 11.The device of claim 1, wherein the second part of the magnetic corecomprises a third element, the third element coupled in contact with thefirst part of the magnetic core.
 12. The device of claim 11, wherein atleast a portion of the third element is disposed between the firstwinding and the second winding.
 13. The device of claim 1, wherein thesecond part of the magnetic core forms an annular structure.
 14. Thedevice of claim 1, further comprising circuitry coupled to the windings.15. The device of claim 14 wherein the first winding and the secondwinding form coupled inductors.
 16. The device of claim 14 wherein thepower conversion device comprises a dual interleaved boost converter.17. The device of claim 16 wherein during operation of the boostconverter, the first winding and the second winding form coupledinductors and during operation cyclical energy storage in the magneticcore is substantially concentrated in the second part of the magneticcore.
 18. The device of claim 1, wherein the first part of the magneticcore comprises a first element and a second element, and a leg of thefirst element and a leg of the second element mate to form the column ofthe first part of the magnetic core.
 19. The device of claim 18, whereinthe first winding is configured to surround the leg of the firstelement, and the second winding is configured to surround the leg of thesecond element.
 20. A power conversion device comprising: a magneticcore; and a plurality of windings surrounding portions of the magneticcore, including a first winding and a second winding magneticallycoupled through the magnetic core; wherein the magnetic core comprises afirst part formed of a first material in direct contact with a secondpart formed of a second material, the first material having a firststiffness and the second material having a second stiffnesssubstantially less than the first stiffness, and the first materialhaving a first magnetic permeability and the second material having asecond magnetic permeability less than the first magnetic permeability;and wherein the first winding and the second winding are magneticallycoupled through the first part of the magnetic core; and wherein thefirst part of the magnetic core comprises a first element and a secondelement, the first element includes a plurality of first legs, thesecond element includes a plurality of second legs, and the first legsand the second legs mate to form the first part of the magnetic core;and wherein the second part of the magnetic core at least partiallyfills the space in between winding material of the first winding andwinding material of the second winding, and wherein the second part ofthe magnetic core is configured to surround at least one of theplurality of first legs or the plurality of second legs.
 21. The deviceof claim 20, wherein the first material has a first magneticpermeability and the second magnetic material has a second magneticpermeability less than the first magnetic permeability.
 22. The deviceof claim 20, wherein the second stiffness is about 1000 times less thanthe first stiffness.
 23. The device of claim 20, wherein the secondstiffness is less than about 100 MPa.
 24. The device of claim 20,wherein the first winding and the second winding form coupled inductors.25. The device of claim 24, wherein the power conversion devicecomprises a dual interleaved boost converter.
 26. A power conversiondevice comprising: a magnetic core; and a plurality of windingssurrounding portions of the magnetic core, including a first winding anda second winding magnetically coupled through the magnetic core; whereinthe magnetic core comprises a first part formed of a first magneticmaterial in direct contact with a second part formed of a secondmagnetic material, the first magnetic material having a first stiffnessand the second magnetic material having a second stiffness substantiallyless than the first stiffness; and wherein the first winding and thesecond winding are magnetically coupled through the first part of themagnetic core; and wherein the second part of the magnetic core at leastpartially fills the space in between winding material of the firstwinding and winding material of the second winding; and wherein, duringoperation of the power conversion device, current in the first windingcreates a first magnetic flux path and current in the second windingcreates a second magnetic flux path different from the first magneticflux path, the first magnetic flux path comprising a first closed patharound a portion of the first winding that passes through the first partof the magnetic core and the second part of the magnetic core without anair gap between the first magnetic material and the second magneticmaterial over the first closed path, and the second magnetic flux pathcomprising a second closed path around a portion of the second windingthat passes through the first part of the magnetic core and the secondpart of the magnetic core without an air gap between the first magneticmaterial and the second magnetic material over the second closed path,and wherein the second part of the magnetic core includes a structure atleast partially surrounding at least a portion of the first part toconcentrate greater cyclical energy storage in the second part of themagnetic core than in the first part of the magnetic core, duringoperation of the power conversion device.
 27. The device of claim 26,wherein the first winding is disposed on a first substrate and thesecond winding is disposed on a second substrate, the magnetic corepassing through openings in the first substrate and the secondsubstrate.
 28. The device of claim 26, wherein the first magnetic fluxpath overlaps with the second magnetic flux path within the second partof the magnetic core.
 29. The device of claim 26, wherein the secondmagnetic material comprises a mechanically deformable magnetic materialconfigured to deform to prevent any air gap between the first magneticmaterial and the second magnetic material over the first and secondclosed paths.
 30. The device of claim 26, wherein the first part of themagnetic core comprises a first element and a second element, and thefirst closed path passes through the first element and the second partof the magnetic core but not the second element, and the second closedpath passes through the second element and the second part of themagnetic core but not the first element.